This invention relates to a process for maximizing the value of light hydrocarbon mixtures containing one or more lower olefins (C.sub.2.sup.=+) such as those typically available in a petroleum refinery, for use in gasoline. Since maximizing the value of the mixtures requires forming a C.sub.3 -C.sub.4 monohydric acyclic alcohol to be used as a reactant in an etherification (or "etheration") reaction, a preferred stream for hydration is a stream containing at least 30% C.sub.3 -C.sub.4 olefins (C.sub.3.sup.= -C.sub.4.sup.=+), and more than 10% by weight (% by wt) of the olefins is propylene or C.sub.3.sup.=+ (propylene and heavier olefins).
Preferred streams of lower olefins to be upgraded consist essentially of predominantly (more than 50% by wt) C.sub.3 -C.sub.4 olefins; or, light naphtha; either of which may sometimes be mixed with a C.sub.4 byproduct containing a C.sub.4.sup.= fraction from an ethylene plant or the like, so that the mixture in the stream has less than 70% by wt, and preferably less than 30% by wt of C.sub.2 -C.sub.5 paraffins. Such streams are generated in cracking and visbreaking units. For example, one available FCC (fluid catalytic cracking) stream may be predominantly C.sub.3 -C.sub.4, and another, a light naphtha stream may be predominantly C.sub.4 -C.sub.5, with a substantial portion of the olefins in each stream being just outside the specified range.
Because the light hydrocarbon mixture usually contains C.sub.3 -C.sub.5.sup.= among which C.sub.3.sup.= together with C.sub.4.sup.= predominate, and in such ,a mixture either C.sub.3.sup.= or C.sub.4.sup.= may predominate, the mixture is referred to herein as a "lower olefin feed stream". The object is to upgrade such feed streams to as high a value for use as gasoline ("gasoline value") as can be justified by the cost of equipment and energy required to upgrade the streams.
More specifically, this overall process relates to a unique scheme for upgrading one or more light olefin-containing feed streams into an ether-rich gasoline product, without resorting to use of any hydrocarbon stream not derived from the feed stream(s), and with a minimum expenditure of energy since liquid-liquid extraction columns are far more energy-efficient than distillation columns. In the basic mode illustrated in FIG. 1, our process does not require a distillation column, though, as illustrated in FIG. 2, distillation columns may be used to tailor the feeds for the gasoline stream used in the extraction column. Of course if a gasoline stream containing C.sub.5 -C.sub.10.sup.= is available in the refinery, and at least 10% of the olefins in such a stream consist of tertalkenes such as isoamylenes, isohexenes and isoheptenes, upgrading the lower olefins is unnecessary.
Much effort has been expended in the prior art to upgrade gasoline by blending methyl, propyl or isopropyl ethers of t-butyl ether with gasoline range hydrocarbons, and to do so by minimizing operating costs. Amongst numerous such processes, examples are provided in U.S. Pat. Nos. 4,664,675 (Class 44/subclass 60) and 4,647,703 to Torck et al (Class 568/subclass 697). Because they chose to etherify gasoline with methanol they could not discover the advantages of etherifying with C.sub.2.sup.+ secondary alcohols, preferably C.sub.3 -C.sub.4 alcohols. Further, they extracted with water, not gasoline.
In U.S. Pat. No. 3,904,384 (Class 44/subclass 56) to Kemp et al, ether-rich gasoline was produced from a single source of C.sub.4 hydrocarbons by cracking to produce propylene and isobutene which are separated. They then hydrate the propylene and etherify the isobutene with the propanol to obtain isopropyl t-butyl ether which is blended with an available stream of gasoline boiling range hydrocarbons. No extraction step is required. In U.S. Pat. No. 4,393,250 (Class 568/subclass 697) to Gottlieb et al, isopropyl alcohol (IPA) was produced from propylene, and the IPA was used to etherify isobutene. They extract their ether-alcohol mixture with water and use a profusion of distillation columns to make the other separations required. We know of no combination of such hydration and etherification processes in which, starting with lower olefins, olefinic gasoline is used both in an etherification reaction, as well as solvent for isopropanol and higher alkanols (C.sub.3.sup.+) used in the reaction.
Our integrated process combines several subordinate processes, referred to as "root processes", in the first one of which a portion of the light olefins are converted by hydration into an aqueous stream (referred to as an "alcoholic effluent") containing a mixture of aliphatic alkanols, a large portion of which mixture is C.sub.3.sup.+ ; in a second root process, the remaining portion of the light olefin stream, or part of it, is oligomerized to yield a gasoline stream (an intermediate or `process` gasoline stream referred to simply as "gasoline stream" for brevity, and to distinguish it from "product gasoline" made by the process) tailored to contain essentially only those aliphatic hydrocarbons having at least 5 carbon atoms (C.sub.5.sup.+), a major portion of which are linear, that is, straight or branched chain olefins (C.sub.5.sup.=+), and a relatively large proportion of these, at least 10% by wt, and preferably at least 30% by wt, are tert-alkenes or isoalkenes; in a third root process the alcohols are extracted from the alcoholic effluent and transferred to the gasoline in a liquid-liquid extraction step; in a fourth root process, the gasoline stream, with a stoichiometric amount of a C.sub.3 or C.sub.4 secondary alcohol, based on the molar amount of tert-alkenes, is reacted in the presence of a solid acidic catalyst, to yield an etherified (or `etherated`) effluent comprising etherated gasoline, unreactive C.sub.5.sup.+ hydrocarbons, unreacted C.sub.5.sup.=+ (from the gasoline stream), and unreacted alcohols, though even less than a stoichiometric amount of secondary alcohol may be used; and, in a fifth root process, the effluent from the etheration reactor comprising remaining unreacted olefins, and alcohols, and unreactive paraffins and other components in the gasoline, and, the etherated gasoline are extracted with water from the etherated effluent to yield the "product gasoline" stream (referred to above) containing an octane-enhancing quantity of di-C.sub.3.sup.+ alkyl ethers in essentially all of which, one alkyl group has at least 3 carbon atoms (C.sub.3.sup.+) and the other at least 5 (C.sub.5.sup.+).
More specifically, unless the tailored C.sub.5.sup.=+ happens to be available, the effectiveness of the overall process is initially predicated upon the double-barreled ability (A) to produce the tailored stream by oligomerizing the light olefin feed stream in an oligomerization zone, such as the reaction zone of a Mobil Olefin to Gasoline ("MOG") or Mobil Olefin to Distillate ("MOD") process, and, (B) to produce an alcoholic effluent, preferably having a major portion by weight of a monohydric alcohol, preferably a secondary alcohol having at least 3 carbon atoms (C.sub.3.sup.+). Thereafter, it so happens that it is not particularly important what the extraction factor for the C.sub.3.sup.+ alcohols is, when they are extracted into the tailored (C.sub.5.sup.=+) gasoline stream, because it is not essential that all the tert-olefins in the gasoline stream be etherated to provide an unexpectedly high octane boost. What is important, is that, a gasoline stream containing C.sub.5 -C.sub.10.sup.=+ is especially well-adapted under the circumstances to extract sufficient secondary alcohols from the alcoholic effluent to provide a mixture which can be etherified without making any separation of components in the feed from the extraction column to the etheration zone.
Extraction with the gasoline stream fortuitously happens to provide the secondary alcohols which are economically desirable, because they are sufficiently reactive under chosen conditions in an etherification reaction zone, to etherify essentially only the tert-olefins in the C.sub.5.sup.=+ stream to yield asymmetrical di-C.sub.3.sup.+ alkyl ethers, and particularly C.sub.3.sup.+ alkyl-t-C.sub.5.sup.+ alkyl ethers, which produce the unexpectedly high boost in octane, relative to methyl or ethyl-t-C.sub.5.sup.+ alkyl ethers, based on the oxygen content (wt % O) contributed by the ethers. Gottlieb et al, supra, used a C.sub.4 fraction containing isobutene to extract isopropyl or sec-butyl alcohol but found it necessary to separate the alcohol from the organic phase by distillation before feeding the alcohol and byproducts (for example, di-isopropylether) to the etheration reactor. The isopropyl alcohol and sec-butyl alcohol are reacted with isobutene to produce isopropyl-t-butyl ether and sec-butyl-t-butyl ether. Even if they had tested the octane boost contributed by their combined t-butyl ethers, they would not have known that t-amyl ethers provided a superior octane boost based on % by wt of oxygen in the etherate. To improve the boost contributed by the t-butyl ethers formed with the C.sub.3 and C.sub.4 alcohols, the Gottlieb et al '250 reference teaches the addition of methyl-t-butyl ether (MTBE).
The ability of lower alkyl ethers to function as octane boosters in gasoline has focused the attention on methanol which is used (i) to etherify isobutylene to yield MTBE, or, (ii) to etherify isoamylenes to yield methyl tert-amyl ether. Methanol is plentiful, and is known to etherify an isoalkene more readily than other secondary or tertiary olefins. By "isoalkene" I refer to a t-monoolefin having the double bond on the tertiary C atom. In the past, methanol was preferred for reaction with C.sub.4 -C.sub.7 isoolefins, as specifically taught in U.S. Pat. No. 4,544,776 (Class 568/-subclass 697) to Osterburg et al., presumably because of the known reactivity of primary alcohols in the etherification of isoolefins.
But we discovered not only that etherification of a (C.sub.5.sup.=+) stream with C.sub.3 -C.sub.5 alcohols, including secondary alcohols, proceeded apace and with gratifying selectivity, but that "base" (C.sub.5.sup.=+) gasoline (RON 93.7; MON 79.1, for example) boosted with an isopropyl ether of the (C.sub.5.sup.=+) isoalkenes has a surprisingly higher octane than it has when boosted with a methyl ether of the (C.sub.5.sup.=+) isoalkenes. This discovery provided the impetus to search for a way to provide a (C.sub.5.sup.=+) stream, and a C.sub.3.sup.+ alcohol stream containing isopropyl and higher alcohols, each stream in condition to be reacted under the appropriate catalytic etherification conditions, and to find a way to recover the product gasoline economically, without resorting to a distillation column.
In the embodiments described hereinafter, it may be desired to operate a MOG reactor in a MOG process, if the amount of distillate range hydrocarbons made in the reactor is to be minimized; however, it may be desired to operate a MOD reactor in a MOD process, if the amount of distillate range hydrocarbons made in the reactor is to be maximized and the distillate recovered, prior to using the gasoline range hydrocarbons for solvent. Reference is made to either the MOG or the MOD mode, or both modes of the process, by designating the "MOG/D" mode. Specific reference to one mode or the other is made by reference to each as being either the MOG or MOD mode.
Under the chosen circumstances, selection of the MOG (or an analogous) process to provide the (C.sub.5.sup.=+) stream was easy, but any inclination to pursue the hydration of an olefin stream to produce the C.sub.3.sup.+ alcohol stream was quickly vitiated by the expense of separating the alcohols and desired secondary alcohols, namely isopropyl or isobutyl and isoamyl alcohols, from an alcoholic effluent which contained a major proportion by weight of water. Separation of the alcohols is avoided in our process, as is separation of the C.sub.5.sup.=+ content of the MOG/D effluent. It may be economical to make such separations by distillation, prior to extraction, to provide the optimum ratio of alcohols and water, and/or optimum concentration of C.sub.5.sup.=+ in the extraction column.
The reaction of methanol with isobutylene, isoamylenes, and higher tertiary olefins, at moderate conditions with a resin catalyst is taught by R. W. Reynolds et al in The Oil and Gas Jour. June 16, 1975; by S. Pecci and T. Floris in Hydroc, Proc. Dec 1977; and, by J. D. Chase et al in The Oil and Gas Jour. Apr 16, 1979 pg 149-152. The preferred catalyst is Amberlyst 15 sulfonic acid resin available from Rohm and Haas Corp. None teaches etherification of C.sub.5.sup.=+ olefins, and particularly C.sub.5 to C.sub.9 isoolefins with C.sub.3.sup.+ alcohols, or isopropyl alcohol, for any reason. There was no reason to expect that the effectiveness of an isopropyl or C.sub.3.sup.+ etherate of a C.sub.5.sup.=+ gasoline should be many times more effective on the basis of its oxygen content (percent by weight O), than a methyl or ethyl etherate of the same C.sub.5.sup.=+ gasoline.